This invention relates to reciprocating piston internal combustion engines and more particularly to a mechanism for controlling maximum cylinder pressures in such engines.
It is known in the art relating to reciprocating piston engines to provide means for limiting maximum cylinder pressures by allowing for some compressive adjustment of the position of the piston relative to its connecting rod. In one prior art arrangement, a preloaded compression spring is provided between the piston and its connecting rod attachment. In another, the piston is mounted on an oil pressure cushion which is connected with a relief valve to control the maximum oil pressure. In both cases, maximum engine cylinder pressure is limited by relative downward movement of the piston on the connecting rod when the maximum desired cylinder pressure is reached.
The provision of mechanisms to limit cylinder pressures can allow the use of higher compression ratios in the cylinder which provide improved efficiency when the engine is operated at lower and medium load conditions. At higher loads, the downward motion of the piston relative to the connecting rod operates to limit maximum cylinder pressures, reducing the rate of pressure rise and resulting in smoother and quieter engine operation without damage from the effects of the higher compression ratio provided in lower load operating conditions. Other mechanisms proposed have included eccentric connecting rod bearings or piston pins which are rotated or oscillated as the piston reciprocates to vary the effective connecting rod length. Means for adjusting the phase of oscillation allows control of maximum piston compression ratio. However the prior art methods for accomplishing these results are considered to excessively complicate piston and connecting rod design.
The present invention utilizes a sandwiched element in the form of an eccentric bearing rotatably mounting the connecting rod on a crankpin of the crankshaft or, alternatively on a wrist pin of the piston. If desired, eccentric bearings could be utilized in both the crankpin and wrist pin locations.
In carrying out the preferred embodiment, the eccentric bearing is mounted in the connecting rod with the center of the outer diameter that rotatably engages the connecting rod offset by a predetermined eccentricity from the inner diameter of the bearing, which engages the crankpin. The eccentricity defines an effective lever arm acting from the center of the connecting rod bore around the center of the crankpin bore.
When load is applied to the piston, the eccentricity tends to force the center of the connecting rod bore from a position longitudinally upward from a lateral axis of the crankpin relative to the connecting rod (a crankpin axis parallel to the lateral axis of the connecting rod bore) toward a position longitudinally downward from the relative lateral crankpin axis. This lowers the position of the piston in the cylinder and reduces the effective compression ratio. The amount of rotational force developed on the eccentric bearing is dependent both on the length and position of the eccentric lever arm as well as on the force applied against the lever arm by the engine cylinder pressures and other forces.
To provide pressure control, the eccentric bearing may be provided with an extending lever or cam which is biased toward a first angular position by a preloaded compression (or tension) spring. The spring rotates the bearing to a first position against a stop, with the connecting rod and attached piston raised to the highest compression ratio position in the cylinder. The compression ratio may remain constant at lower loads and above until a predetermined force generated by cylinder pressure is reached.
At this point, the force acting on the eccentric lever arm or eccentricity of the bearing overcomes the restraining force of the spring, causing the eccentric bearing to rotate in the connecting rod to move the eccentric end of the rod outward, away from the cylinder. This motion effectively shortens the length of the connecting rod and moves the piston downward relative to the connected crankpin. In this manner, the effective compression ratio of the cylinder is temporarily reduced and the pressure developed in the cylinder is controlled by the mechanical mechanism.